In the New Testament the Greek word for Spirit is Pneuma. The study of the Spirit is thus referred to as "Pneumatology." There aren't many literary works that deal specifically with Pneumatology except for "Word Studies on the Holy Spirit by E.W. Bullinger" which is a highly recommended reading. In the book of Joel as well as throughout the Old Testament we can observe and meditate upon how God spoke to us beforehand about an outpouring of the Holy Spirit upon all mankind. The Jewish people, God's chosen ones, were somewhat misguided in their assumption that God's plan of salvation was exclusive to them, which would disqualify the rest of all humanity. God had greater things in mind well before the process of creation had begun, which included all of God's creation Beginning in the book of Genesis we see a blueprint being laid out that laid a foundation for the redemption of all mankind, so that nothing that took place was accidental or coincidental. Everything that has ever happened and is happening now or will ever happen is by design and God is at the helm of a ship named Salvation, rescuing all that are drowning in a sea of sin, and navigating towards the destination that leads to eternal life. This vessel is powered by the Holy Ghost which is the active force that is at work in all believers since that day of Pentecost mentioned in the first chapters of the book of Acts. This is the gift of God that was promised to us in scripture, which was confirmed by Christ Himself, and poured out in that upper room in days past, to usher in the manifestation of the fulfillment of the promise that we have partaken of and have enjoined ourselves to, until all things written in the scriptures are fulfilled.